1. Field of the Invention
This invention relates to a mirror having reflective coatings on a first surface and an opposite second surface, and more particularly, to a solar mirror having an opaque reflecting coating on a second surface of a transparent substrate to reflect light passing through the substrate and a transparent reflecting coating on the opposite first surface of the transparent substrate to reflect wavelengths in selected ranges of the electromagnetic spectrum that are absorbable by the substrate.
2. Discussion of the Technical Challenge
At the present time, there is interest to increase the efficiency of solar collectors, e.g. and not limiting to the discussion, improve the efficiency of solar mirrors, e.g. flat solar reflecting mirrors and shaped solar reflecting mirrors, used to reflect the sun's rays to a converting device. The converting device is usually of the type known in the art to convert the sun's energy to another form of energy, e.g. electric energy and/or thermal energy. In general and not limiting to the discussion, the solar mirror can be a primary mirror or a secondary mirror. The term “primary mirror” as used herein is a mirror on which solar rays are first reflected, and the term “secondary mirror” as used herein is a mirror on which reflected solar rays are re-reflected, e.g. to another solar mirror, or to a receiving element or receiver. The reflected solar rays incident on the secondary mirror can be reflected from a primary mirror or from another secondary mirror. The receiving element, or receiver, can include, but is not limited to, photovoltaic devices or a tube containing a fluid.
In general but not limiting to the discussion, the primary mirror is a shaped mirror, e.g. a parabolic, or cylindrical, shaped mirror having an opaque solar reflective coating, e.g. a silver coating on the convex surface or second surface of a shaped transparent substrate. The secondary mirror can be a shaped mirror or a flat mirror having the opaque solar reflective coating on a surface of a shaped or flat transparent substrate. Usually, the secondary mirror is a flat mirror having the reflective coating on the back surface or the second surface of a flat or lens shaped transparent substrate. In practice, the solar rays are incident on the first surface or concave surface of the primary mirror. A portion of the sun's rays are reflected from the first surface of the shaped mirror toward the receiver, or a secondary mirror, and a portion of the sun's rays pass through the substrate and are reflected by the opaque reflective coating back through the transparent substrate toward the receiver or the secondary mirror. In the instance when the sun's rays are reflected toward a secondary mirror, the reflected sun's rays from the primary mirror are incident on the secondary mirror and reflected by the secondary mirror to the receiver, or toward another secondary mirror. A more detailed discussion of primary and secondary solar reflecting mirrors is presented in U.S. patent application Ser. No. 12/709,045 filed on Feb. 19, 2010 and titled SOLAR REFLECTING MIRROR HAVING A PROTECTIVE COATING AND METHOD OF MAKING SAME, which document in its entirety is hereby incorporated by reference.
The transparent substrate of the primary and the secondary mirrors is usually made of soda-lime-silica glass because of the high yield in shaping a flat piece of soda-lime-silica glass into a parabolic shaped substrate; the low cost of making soda-lime-silica glass, and the high yield and low cost of applying a solar reflective coating on a surface of a flat piece or shaped piece of soda-lime-silica glass. Although soda-lime-silica glass is an acceptable material for the substrates for the solar mirrors, there are limitations. More particularly, a commercial grade soda-lime-silica glass is made of batch materials that include ingredients that absorb selected wavelengths of the electromagnetic spectrum. For example and not limiting to the discussion, a commercial grade of batch materials to make soda-lime-silica glass usually has at least 0.04 weight percent of iron oxides, namely ferric oxide (Fe2O3) and ferrous oxide (FeO). The ferric oxide has its absorption in the wavelength range of 300 to 400 nanometers (“nm”) of the electromagnetic spectrum, and the ferrous oxide has its absorption in the wavelength range of 780-1550 nm of the electromagnetic spectrum and its peak absorption in the wavelength range 1000-1200 nm of the electromagnetic spectrum. The absorption by the ferric oxide in the 300-400 nm range, and by the ferrous oxide in the 780-1550 nm range, of the electromagnetic spectrum reduces the amount of solar energy incident on the converting device.
As is appreciated by those skilled in the art, a purer grade of soda-lime-silica glass batch materials having reduced weight percents of iron oxides are available. For example, soda-lime-silica glasses having less than 0.04 weight percent of iron oxides are disclosed in U.S. patent application Ser. No. 12/275,264 filed Nov. 21, 2008 and U.S. Pat. No. 5,030,594, which documents in their entirety are incorporated herein by reference. PPG Industries, Inc. sells such glasses under the trademarks STARPHIRE and SOLARPHIRE PV.
Unfortunately, the cost of batch materials for making soda-lime-silica glasses having less than 0.04 weight percent of iron oxides is two to three times more expensive than the cost of the batch materials for making soda-lime-silica glasses having more than 0.04 weight percent of iron oxides. As can now be appreciated, it would be advantageous to provide a solar reflecting mirror having a soda-lime-silica glass substrate having greater than 0.04 weight percent of iron oxides and having reduced absorption of wavelengths in selected ranges of the electromagnetic spectrum, e.g. in the wavelength ranges of 300-400 nm, and 780-1550 nm, of the electromagnetic spectrum.